Even though polyvinylidene fluoride (PVDF), a polymer used for membrane production, is thermoresistant up to about 300.degree. C., it develops extremely hydrophobic properties in aqueous systems (EP-A2-0 249 513), which result in poor membrane wettability in aqueous systems. Thus, it was tried to mold polymer solutions into membranes, which contain a hydrophilic polymer, such as polyvinylpyrrolidone (PVP), in addition to the hydrophobic PVDF (EP-A1-0 012 557). However, the PVP amounts needed for the resulting membranes to show sufficient hydrophilic properties are so high that the membranes no longer have the necessary mechanical stability. Furthermore, the water-soluble PVP is washed out of the membrane by water.
It was tried to eliminate this drawback by grafting the PVP onto PVDF membranes by means of radiation rich in energy (A. Niemoller and G. Ellinghorst, Die Angewandte Makromolekulare Chemie 151, No. 2489, 49-67 (1987)). However, as is well known, grafting represents a technically onerous process and also results in a non-uniform distribution of the grafted polymer. This step of grafting PVP onto PVDF membranes leads to an increase in trans-membrane flow by a factor of 3 along with hydrophilicity (water uptake increase of 45% by weight). However, this effect is accompanied by a very special, hardly adjustable, small degree of grafting. If it is exceeded, opposite effects will occur.
It was also tried to increase the hydrophilicity of these membranes and thus their trans-membrane flow by mixing PVDF with polyvinyl acetate and subsequently saponifying the mixture to form water-soluble polyvinyl alcohols (EP-A1-0 012 557). Thus, membranes containing 65% by weight of polyvinyl acetate, based upon the total polymer content of the membrane, have a flow of 41 lm.sup.-2 h.sup.-1 /345 kPa; i.e. 11.8 lm.sup.-2 h.sup.-1 bar.sup.-1. However, if less than 40% by weight of polyvinyl acetate, based upon the total polymer content of the membrane, are added, membranes will result which have poorer trans-membrane flows than those made of pure PVDF.
Therefore, it is the object of this invention to provide polymer membranes having high thermal and mechanical load-bearing capacity as well as a high degree of hydrophilicity, great trans-membrane flows and a retention capacity of 95% for substances having molecular weights between 700 and 1400.